From what you write, it really sounds like the display driver is faulty (did you observe more of "missing segments" or "shape distortions" in the handwritten trace?).
Well, this is not something new, the drivers in eos are far from being "perfect". Anyway, to get it properly working I had to change the display colors from 24 bits to 16 bits and change in the ritePen settings the pen width to the thinner possible. After these changes the program worked fine.